Yeah, they are prone to leaking around the top up lid, not by a lot, but the sensor is really sensitive. Just keep your eye on the level every now and then, or try a bit of clear silicone, if you can identify the leak.
Stew
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Followup; the coolant level is fine, but the light keeps coming on. There seem be sensor wires running into the clear coolant reservoir. If the level in the clear tank is fine, should I worry ?
Or is there another sensor at the bottom of the block or something.... that might be airlocked or something...
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If the level in the clear tank is fine, it sounds like the sensor itself is knackered. My understanding of the warning lamp, is that it only comes on when the water level is low, therefore if yours isn't, then you have a faulty sensor.